Ordered Mesoporous Silica to Study the Preparation of Ni/SiO₂ ex Nitrate Catalysts: Impregnation, Drying, and Thermal Treatments

Abstract

In this contribution, we investigated the preparation of Ni/SiO 2 catalysts with aqueous [Ni(OH 2 ) 6 ](NO 3 ) 2 solutions via the impregnation and drying method using ordered mesoporous silica SBA-15 (mesopore diameter of 9 nm) as model support to study each step in the preparation: impregnation, drying, calcination, and reduction. After impregnation, not all the mesopores of SBA-15 appeared filled with precursor solution. Consecutive drying led to formation of 9 nm Ni 3 (NO 3 ) 2 (OH) 4 crystallites exclusively within the mesopores. During air calcination, severe sintering and redistribution took place, resulting in a low NiO dispersion, including large NiO crystals outside of the mesopores and rodlike NiO particles inside the mesopores. The degree of sintering depended on the concentration of Ni 3 (NO 3 ) 2 (OH) 4 decomposition products (NO 2 , N 2 O, O 2 and H 2 O), and in particular NO 2 and O 2 were found to promote sintering and redistribution. Therefore, maintaining low concentrations of the latter components during the thermal nitrate decomposition is advocated, which was achieved by carrying out the treatment in the presence of H 2 . The latter treatment prevented formation of NO 2 /O 2 as decomposition products, moderated the decomposition rate of Ni 3 (NO 3 ) 2 (OH) 4 into NiO as observed from in situ XRD experiments, and led to NiO particles of 3 nm on average at a loading of 20 wt % Ni/SiO 2 .

How nitric oxide affects the decomposition of supported nickel nitrate to arrive at highly dispersed catalysts

“…A non-uniform distribution of NiO over the mesopores was found in all three samples. The presence of unfilled pores with ordered mesoporous silica supported metal (oxide) catalysts has been observed by others too [46,47] and is presumably related to non-uniform wetting during impregnation [48]. The NiO/SBA-15 sample obtained through air calcination contained both large NiO particles outside the mesopores and particles retained inside the pores.…”

“…7). It has already [48,61]. Moreover, we recently discovered that N 2 O also has an advantageous effect [62].…”

Copper Nitrate Redispersion To Arrive at Highly Active Silica-Supported Copper Catalysts

“…2,23À25 A number of methods to counter agglomeration of transition metal nitrates such as nickel, cobalt, and copper nitrate have been reported, including replacement of the air calcination by glow discharge plasma-assisted nitrate decomposition 26 and direct reduction of the metal nitrate to the metal, omitting air thermal treatment. 5,25,27,28 Another approach is fast removal of the decomposition gases via high space velocities or evacuation. 24, 29 We have previously reported that agglomeration of cobalt and nickel nitrate can effectively be prevented by replacing traditional air calcination with thermal treatment in 1% (v/v) NO/He flow, resulting in high NiO and Co 3 O 4 dispersions.…”

Gas‐Phase Model Studies Relevant to the Decomposition of Transition‐Metal Nitrates M(NO3)2 (M = Co, Ni) into Metal–Oxo Species

“…Thus, metal nitrates have indeed been used as precursors for the generation of metal oxides and due to the oxidizing properties of nitrates also high oxidation states may be maintained in the products resulting from thermolysis. [1,2] Recently, controlled pyrolysis of metal nitrates has also been proposed as a new route for the preparation of metal oxide nanoparticles, [3,4] and NiO/ SiO 2 catalysts with narrow distributions of particle sizes and particle-particle distances have also been obtained. [5,6] The mechanistic knowledge about the details of the thermal decomposition of nitrates is limited, however, mostly due to the complex radical chain mechanisms that are involved ions containing formal M III .…”